THE OUTER PLANETS.
Jupiter, the greatest planet of the Solar System, has perhaps been more
persistently studied by astronomers than any other. In the early
nineteenth century the prevalent idea was that Jupiter was a world
similar to the Earth, only much larger,—a view held by Herschel and
other famous astronomers, and put forward by Brewster in ‘More Worlds
than One.’ This view prevailed for many years, although Buffon in 1778,
and Kant in 1785, had stated their belief in the idea that Jupiter was
still in a state of great heat—in fact, that the great planet was a
semi-sun. This idea, however, was long in being adopted by astronomers,
and very little attention was paid to Nasmyth’s expression of the same
opinion in 1853. The older view still held the field—namely, that the
belts of Jupiter represented trade-winds, and that a world similar to
the terrestrial lay below the Jovian clouds. In 1860 _George Philip
Bond_ (1826-1865), director of the Harvard Observatory, found from
experiments that Jupiter seemed to give out more light than it received,
but he did not dare to suggest that Jupiter was self-luminous,
considering that the inherent light might result from Jovian auroras.
In 1865 Zöllner showed that the rapid motions of the cloud-belts on both
Jupiter and Saturn indicated a high internal temperature. At the
distance of Jupiter sun-heat is only one twenty-seventh as great as on
the Earth, and would be quite incapable of forming clouds many times
denser than those on the Earth. In 1871 Zöllner drew attention to the
equatorial acceleration of Jupiter, analogous to the same phenomenon on
the Sun. In 1870 these opinions of Zöllner’s were adopted and supported
by Proctor in his ‘Other Worlds than Ours.’ In his subsequent volumes
Proctor did much to popularise the idea, which is now accepted all over
the astronomical world.
During the century many valuable observations on Jupiter were made by
numerous observers, among them Airy, Mädler, Webb, Schmidt, and others.
Much time was devoted to the accurate determination of the rotation
period, which was fixed at 9 hours 55 minutes 36·56 seconds by Denning
in observations from 1880 to 1903. No really important discovery was
made till 1878, when Niesten at Brussels discovered the “great red
spot,” a ruddy object 25,000 miles long by 7000 broad, attached to a
white zone beneath the southern equatorial belt. This remarkable object
has been observed ever since. In 1879 its colour was brick-red and very
conspicuous, but it soon began to fade, and Riccó’s observation at
Palermo in 1883 was thought to be the last. After some months, however,
it brightened up, and, notwithstanding changes of form and colour, it is
still visible, a permanent feature of the Jovian disc. In 1879 a group
of “faculæ,” similar to those on the Sun, was observed at Moscow by
_Theodor Alexandrovitch Brédikhine_ (1831-1904), and at Potsdam by
_Wilhelm Oswald Lohse_ (born 1845). It was soon observed that the
rotation period, as determined from the great red spot, was not
constant, but continually increasing. A white spot in the vicinity
completed its rotation in 5½ minutes less, indicating the differences of
rotation on Jupiter.
The great red spot has been observed since its discovery by Denning at
Bristol and _George Hough_ (born 1836) at Chicago. Twenty-eight years of
observation have not solved the mystery of its nature. The researches
made on it, in the words of Miss Clerke, “afforded grounds only for
negative conclusions as to its nature. It certainly did not represent
the outpourings of a Jovian volcano; it was in no sense attached to the
Jovian soil—if the phrase have any application to the planet; it was not
a mere disclosure of a glowing mass elsewhere seethed over by rolling
vapours.”
In 1870 _Arthur Cowper Ranyard_ (1845-1894), the well-known English
astronomer, began to collect records of unusual phenomena on the Jovian
disc to see if any period regulated their appearance. He came to the
conclusion that, on the whole, there was harmony between the markings on
Jupiter and the eleven-year period on the Sun. The theory of inherent
light in Jupiter, however, has not been confirmed. The great planet was
examined spectroscopically by Huggins from 1862 to 1864, and by Vogel
from 1871 to 1873. The spectrum showed, in addition to the lines of
reflected sunlight, some lines indicating aqueous vapour, and others
which have not been identified with any terrestrial substance. A
photographic study of the spectrum of Jupiter was made at the Lowell
Observatory by Slipher in 1904, probably the most exhaustive
investigation on the subject. The spectroscope has, however, given
little support to the theory of inherent light, and “we are driven to
conclude that native emissions from Jupiter’s visible surface are local
and fitful, not permanent and general.”
Herschel’s idea, that the rotations of the four satellites of Jupiter
were coincident with their revolutions, has on the whole been confirmed
by recent researches, although in the case of the two near satellites
(Io and Europa) W. H. Pickering’s observations in 1893 indicated shorter
rotation periods. There is much to learn regarding the geography of the
satellites, although in 1891 Schaeberle and Campbell at the Lick
Observatory observed belts on the surface of Ganymede, the third
satellite analogous to those on Jupiter. Surface-markings on the
satellites have also been seen by Barnard at the Lick Observatory, and
by Douglass at Flagstaff.
Since the time of Galileo no addition had been made to the system of
satellites revolving round Jupiter. Profound surprise was created,
therefore, by the announcement of the discovery of a fifth satellite by
Barnard at the Lick Observatory, on September 9, 1892. The satellite,
one of the faintest of telescopic objects, was discovered with the great
36-inch telescope, and its existence was soon confirmed by _Andrew
Anslie Common_ (1841-1903), with his great 5-foot reflector at Ealing,
near London. The new satellite was found by Barnard to revolve round
Jupiter in 11 hours 57 minutes at a mean distance of 112,000 miles.
Although the existence of other satellites of Jupiter was predicted by
Sir _Robert Stawell Ball_ (born 1840) soon after the discovery of the
fifth, much surprise was created by the announcement, in January 1905,
that a sixth satellite had been discovered by Perrine, who, in the
following month, announced the discovery of a seventh. These discoveries
were made by photography, the objects being very faint. The periods of
revolution were found to be 242 days and 200 days for the sixth and
seventh satellites respectively, the mean distances being 6,968,000 and
6,136,000 miles. It is possible that they may belong to a zone of
asteroidal satellites. In fact, the fifth moon may belong to a similar
zone, so that Jupiter may have two asteroidal zones; but this is
anticipating future discovery.
A particular charm has always attached itself to the study of Saturn,
the ringed planet. The magnificent system of rings has for two and a
half centuries been the object of wonder and admiration in the Solar
System, and accordingly they have been exhaustively studied by many
eminent observers. While observing the two bright rings of Saturn on
June 10, 1838, Galle noticed what Miss Clerke calls “a veil-like
extension of the lucid ring across half the dark space separating it
from the planet.” No attention, however, was paid to Galle’s
observation. On November 15, 1850, _William Cranch Bond_ (1789-1859), of
the Harvard Observatory in Massachusetts, discovered the same phenomenon
under its true form—that of a dusky ring interior to the more brilliant
one. A fortnight later, before the news of Bond’s observation, Dawes
made the same discovery independently at Wateringbury in England. This
ring is known as the dusky or “crape” ring.
The discovery of the dusky ring brought to the front the problem of the
composition of the ring-system. Laplace and Herschel considered the
rings to be solid, but this was denied in 1848 by _Edouard Roche_
(1820-1880), who believed them to consist of small particles, and in
1851 by G. P. Bond, who asserted that the variations in the appearance
of the system were sufficient to negative the idea of their solidity;
but he suggested that the rings were fluid. In 1857 the question was
taken up by the Scottish physicist, _James Clerk-Maxwell_ (1831-1879),
who proved by mathematical calculation that the rings could be neither
solid nor fluid, but were due to an aggregation of small particles, so
closely crowded together as to present the appearance of a continuous
whole. Clerk-Maxwell’s explanation—which had been suggested by the
younger Cassini in 1715, and by Thomas Wright in 1750—was at once
adopted, and has since been proved by observation. In 1888 _Hugo
Seeliger_ (born 1849), director of the Munich Observatory, showed from
photometric observations the correctness of the satellite-theory; while
Barnard in 1889 witnessed an eclipse of the satellite Japetus by the
dusky ring. The satellite did not disappear, but was seen with perfect
distinctness. The final demonstration of the meteoric nature of the
rings was made by Keeler at the Alleghany Observatory in 1895, with the
aid of the spectroscope. By means of Doppler’s principle, he found that
the inner edge of the ring revolved in a much shorter time than the
outer, proving conclusively that they could not be solid. This was
confirmed by the observations of Campbell at Mount Hamilton, _Henri
Deslandres_ at Meudon, and Bélopolsky at Pulkowa.
In 1851 a startling theory regarding Saturn’s rings was put forward by
the famous _Otto Wilhelm von Struve_ (1819-1905). Comparing his
measurements on the rings made at Pulkowa in 1850 and 1851 with those of
other astronomers for the past two hundred years, he reached the
conclusion that the inner diameter of the ring was decreasing at the
rate of sixty miles a-year, and that the bodies composing the rings were
being drawn closer to the planet. Accordingly, Struve calculated that
only three centuries would be required to bring about the precipitation
of the ring-system on to the globe of Saturn. In 1881 and 1882 Struve,
expecting a further decrease, made another series of measures, but these
did not confirm his theory, which was accordingly abandoned.
The study of the globe of Saturn has made less progress than that of the
rings. The surface of the planet had been known since before the time of
Herschel to be covered with belts, but as spots seldom appear on Saturn,
only one determination of the rotation period had been made, that by
Herschel. Much interest was aroused, therefore, by the discovery, by
Hall, at Washington, on December 7, 1876, of a bright equatorial spot.
Hall studied this spot during sixty rotations of the planet, determining
the period as 10 hours 14 minutes 24 seconds. This was confirmed by
Denning in 1891, and by _Stanley Williams_, an English observer, in the
same year. On June 16, 1903, Barnard, at the Yerkes Observatory,
discovered a bright spot, from which he deduced a rotation period of 10
hours 39 minutes,—a period considerably longer than that found by Hall.
In the same year various spots on Saturn were observed by Denning, who
found a period of 10 hours 37 minutes 56·4 seconds, and at Barcelona by
_José Comas Sola_, now director of the Observatory there, who may be
considered Spain’s leading astronomer. The result of these observations
has been to show that the spots on Saturn have probably a proper motion
of their own, apart from the rotation of the planet. As to the spectrum
of Saturn, little has been learned. It closely resembles that of
Jupiter. In 1867 Janssen, observing from the summit of Mount Etna, found
traces of aqueous vapour in the planet’s atmosphere.
In the chapters on Herschel we have seen that he discovered the sixth
and seventh satellites of Saturn. The next discovery was made on
September 19, 1848, by W. C. Bond, at Harvard, Massachusetts, and
independently by _William Lassell_ (1799-1880), at Starfield, near
Liverpool. The new satellite received the name of Hyperion, and was
found to be situated at a distance of about 946,000 miles from Saturn.
Its small size led Sir John Herschel to the idea that it might be an
asteroidal satellite. Fifty years elapsed before another satellite of
Saturn was discovered. In 1888 W. H. Pickering commenced a photographic
search for new satellites of the planet. At last, on developing some
photographs of Saturn, taken on August 16, 17, and 18, 1898, he found
traces of a new satellite which he named “Phœbe.” But, as the satellite
was not seen or photographed again for some years, many astronomers were
sceptical as to its existence. However, photographs taken in 1900, 1901,
and 1902 revealed the satellite, which was again photographed in 1904,
and seen visually by Barnard in the same year with the 40-inch Yerkes
telescope. At that time the discoverer brought out the amazing fact that
the motion of the satellite is retrograde—a fact which he attempts to
explain by a new theory of the former rotation of Saturn. He likewise
demonstrated that its distance from Saturn varied from 6,120,000 to
9,740,000 miles. Early in 1905 Pickering announced the discovery of a
tenth satellite of Saturn, which received the name of Themis, with a
period and mean distance nearly similar to Hyperion, so that Sir John
Herschel’s idea of Hyperion being an asteroidal satellite is being
confirmed after a lapse of half a century.
If little is known of the globe of Saturn, still less is known regarding
Uranus. Dusky bands resembling those of Jupiter were observed by Young
at Princeton in 1883. In the following year Paul and Prosper Henry
discerned at Paris two grey parallel lines on the disc of the planet.
This was confirmed by the observations of Perrotin at Nice, which also
indicated rotation in a period of ten hours. In 1890 Perrotin again took
up the study and re-observed the dark bands. On the other hand, no
definite results regarding the planet were obtained by the Lick
observers in 1889 and 1890. Measurements of the planet by Young,
Schiaparelli, Perrotin, and others indicate a considerable polar
compression. The spectrum of the planet has been studied by Secchi,
Huggins, Vogel, Keeler, Slipher, and others. The spectrum shows six
bands of original absorption, a line of hydrogen, which, says Miss
Clerke, “implies accordingly the presence of free hydrogen in the
Uranian atmosphere, where a temperature must thus prevail sufficiently
high to reduce water to its constituent elements.” From a photographic
study of the spectrum at the Lowell Observatory in 1904, Slipher
observed a line corresponding to that of helium, indicating the presence
of that element in the planet’s atmosphere.
Herschel left our knowledge of the Uranian satellites in a very
uncertain state. The two outer satellites, Titania and Oberon, were
rediscovered in 1828 by his son, but the other four, which he was
believed to have discovered, were never seen again. In 1847 two inner
satellites, Ariel and Umbriel, were discovered by Lassell and Otto
Struve respectively, their existence being finally confirmed by
Lassell’s observations in 1851.
After the discovery of Uranus by Herschel, mathematical astronomers
determined its orbit and calculated its position in the future. _Alexis
Bouvard_, the calculating partner of Laplace, published tables of the
planet’s motions, founded on observations made by various astronomers
who had considered it a star before its discovery by Herschel; but as
the planet was not in the exact position which Bouvard predicted, he
rejected the earlier observations altogether. For a few years the planet
conformed to the Frenchman’s predictions, but shortly afterwards it was
again observed to move in an irregular manner, and the discrepancy
between observation and the calculations of mathematicians became
intolerable. Did the law of gravitation not hold good for the frontiers
of the Solar System? Gradually astronomers arrived at the conclusion
that Uranus was being attracted off its course by the influence of an
unseen body, an exterior planet. Bouvard himself was one of the first to
make the suggestion, but died before the planet was discovered. An
English amateur, the Rev. _T. J. Hussey_, resolved to make, in 1834, a
determination of the place of the unseen body, but found his powers
inadequate; and in 1840 Bessel laid his plans for an investigation of
the problem, but failing health prevented him carrying out his design.
In 1841 a student at the University of Cambridge resolved to grapple
with the problem. John Couch Adams, born at Lidcot in Cornwall in 1819,
entered in 1839 the University of Cambridge, where he graduated in 1843.
From 1858 Professor of Astronomy at Cambridge, and from 1861 director of
the Observatory, he died on January 21, 1892, after a life spent in
devotion to mathematical astronomy. In 1843, on taking his degree, he
commenced the investigation of the orbit of Uranus. For two years he
worked at the difficult question, and by September 1845 came to the
conclusion that a planet revolving at a certain distance beyond Uranus
would produce the observed irregularities. He handed to _James Challis_
(1803-1882), the director of the Cambridge Observatory, a paper
containing the elements of what was named by Adams “the new planet.” On
October 21 of the same year he visited Greenwich Observatory, and left a
paper containing the elements of the planet, and approximately fixing
its position in the heavens. But the Astronomer-Royal of England, Sir
_George Biddell Airy_ (1801-1892), had little faith in the calculations
of the young mathematician. He always considered the correctness of a
distant mathematical result to be a subject rather of moral than of
mathematical evidence: in fact, regarding Uranus, the Astronomer-Royal
almost called in question the correctness of the law of gravitation.
Besides, the novelty of the investigations aroused scepticism, and the
fact that Adams was a young man, and inexperienced, went against Airy’s
acceptance of the theory. However, he wrote to Adams questioning him on
the soundness of his idea. Adams thought the matter trivial, and did not
reply. Airy, therefore, took no interest in the investigations, and no
steps were taken to search for the unseen planet. Meanwhile the Rev. W.
R. Dawes happened to see Adams’ papers lying at Greenwich, and wrote to
his friend, the well-known astronomer Lassell, who was in possession of
a very fine reflector, erected at his residence near Liverpool, asking
him to search for the planet. But Lassell was suffering from a sprained
ankle, and Dawes’ letter was accidentally destroyed by a housemaid. So
Adams’ theory remained in obscurity.
The question now came under the notice of _François Jean Dominique
Arago_ (1786-1853), the director of the Paris Observatory. He recognised
in a young friend of his a rising genius, who was competent to solve the
problem. Urban Jean Joseph Le Verrier, born at Saint Lo, in Normandy, in
1811, became in 1837 astronomical teacher in the École Polytechnique,
and in 1853 director of the Paris Observatory. In consequence of
differences with his staff he was obliged, in 1870, to resign from this
position, but two years later was restored to the post, which he held
till his death on September 23, 1877.
In 1845, ignorant of the fact that Adams had already solved the problem,
Le Verrier began his investigations of the irregular motions of Uranus.
In a memoir communicated to the Academy of Sciences in November of that
year, he demonstrated that no known causes could produce these
disturbances. In a second memoir, dated June 1, 1846, he announced that
an exterior planet alone could produce these effects. But Le Verrier had
now before him the difficult task of assigning an approximate position
to the unseen body, so that it might be telescopically discovered. After
much calculation Le Verrier, in his third memoir (August 31, 1846),
assigned to the planet a position in the constellation Aquarius.
Meanwhile one of Le Verrier’s papers happened to reach Airy. Seeing its
resemblance to Adams’ papers, which had been lying on his desk for
months, his scepticism vanished, and he suggested to Challis that the
planet should be searched for with the Cambridge equatorial. In July
1846 the search was commenced. The planet was actually observed on
August 4 and 12, but, owing to the absence of star maps, it was not
recognised. “After four days of observing,” he wrote to Airy, “the
planet was in my grasp if I had only examined or mapped the
observations.”
Le Verrier wrote to Encke, the illustrious director of the Berlin
Observatory, desiring him to make a telescopic search for a planetary
object situated in the constellation Aquarius, as bright as a star of
the eighth magnitude and possessed of a visible disc. “Look where I tell
you,” wrote the French astronomer, “and you will see an object such as I
describe.” Encke ordered his two assistants, Galle and D’Arrest, to make
a search on the night of September 23, 1846. In a few hours Galle
observed an object not marked in the star-maps of the Berlin
Observatory, which had been recently published. The following night
sufficed to show that the object was in motion, and was therefore a new
planet. On September 29 Challis found the planet at Cambridge, but he
was too late, as the priority of the discovery was now lost to Adams.
The planet received the name of “Neptune.”
For some time, indeed, it appeared as if the French astronomer alone was
to receive the honour of the discovery. But on October 3, 1846, a letter
from Sir John Herschel appeared in the ‘Athenæum’ in which he referred
to the discovery made by Adams. The French scientists were extremely
jealous. Indeed, Arago actually declared that, when Neptune was under
discussion, the entire honour should go to Le Verrier, and the name of
Adams should not even be mentioned,—Arago’s line of reasoning being that
it was not the man who first made a discovery who should receive the
credit, but he who first made it public. However, the credit of the
discovery is now given equally to Adams and Le Verrier, both of whom are
regarded as among the greatest of astronomers.
Only a fortnight after the discovery of Neptune, the astronomer Lassell
observed a satellite to the distant planet on October 10, 1846. This
discovery was confirmed in July 1847 by the discoverer himself, and
shortly afterwards by Bond and Otto Struve. Regarding the globe of
Neptune, we know practically nothing. No markings of any kind have been
observed on its surface. However, in 1883 and 1884, _Maxwell Hall_, an
astronomer in Jamaica, noticed certain variations of brilliance which
suggested a rotation-period of eight hours, but this was not confirmed
by any other astronomer. The spectrum of Neptune has been investigated
by various observers, who have found it to be similar to that of Uranus.
The existence of a trans-Neptunian planet has been suspected by many
astronomers. In November 1879 the first idea of its existence was thrown
out by Flammarion in his ‘Popular Astronomy.’ Flammarion noticed that
all the periodical comets in the Solar System have their aphelion near
the orbit of a planet. Thus Jupiter owns about eighteen comets; Saturn
owns one, and probably two; Uranus two or three; and Neptune six. The
third comet of 1862, however, along with the August meteors, goes
farther out than the orbit of Neptune. Accordingly, Flammarion suggested
the existence of a great planet, assigning it a period of 330 years and
a distance of 4000 millions of miles.
Two independent investigators, _David Peck Todd_ (born 1855) in America
and _George Forbes_ in Scotland, have since undertaken to find the
planet. Todd, utilising the “residual perturbations” of Uranus, assigned
a period of 375 years for his planet. Forbes, on the other hand, working
from the comet theory, stated his belief in the existence of two planets
with periods of 1000 and 5000 years respectively. In October 1901 he
computed the position of the new planet on the celestial sphere, fixing
its position in the constellation Libra, and computing its size to be
greater than Jupiter. A search was made by means of photography, in
1902, but without success. Nevertheless, astronomers are pretty
confident of the existence of one or more trans-Neptunian planets.
Lowell is very definite on this subject when he says in regard to meteor
groups, “The Perseids and the Lyrids go out to meet the unknown planet,
which circles at a distance of about forty-five astronomical units from
the Sun. It may seem strange to speak thus confidently of what no mortal
eye has seen, but the finger of the sign-board of phenomena points so
clearly as to justify the definite article. The eye of analysis has
already suspected the invisible.”